This system is a combination of Roman aquaducts, log-flumes and
cylindrical, unpowered zeppelins. Picture a series of columns
supporting an elevated network of U-profiled chutes (preferably of
transparent plexiglass) which are partially filled with sulphur
hexaflouride. As this medium is approximately
6 times the density
of air, you wouldn't be required to tap into diminishing supplies of
helium or use dangerous hydrogen gasses to inflate your freight
airship but could use simple air.

A network of such flumes would crisscross the country and be
constructed in such a way that the SF6 would flow down the
channels in a manner analogous to water in an aquaduct or log-
flume. Build as many cylindrical zeppelins as you like for
transporting non time sensitive bulk goods such as timber, ores
and grains throughout the network. When the terrain no longer
permits a downstream flow, the zeppelins are towed up the
sloped chute to gain elevation and potential energy in the same
manner as the "logs" are returned to the top in the log-flume rides
found at amusement parks. The sulphur hexaflouride is also
pumped uphill in either gaseous form or, if chilled below -65C,
liquid form.

The zeppelin network would be slow but could run autonomously
with the switching junctions working similar to shipping locks. I
could imagine there would be a boutique market for passengers
seeking to see the countryside at a leisurely pace who would book
staterooms in the upper half of the freight zeppelins (ABOVE the
SF6 and the sides of the chutes so they can do that breathing thing
which high-paying passengers demand these days).

A major difficulty is that sulphur hexaflouride is a major and long-
lasting greenhouse gas. One way to mitigate the damage done is
to source your gas from sulphur hexaflouride which is in, or at
least going into, the atmosphere already such as from volcanic
outgassings. Another way is to encapsulate the entire system by
using tubes instead of chutes but it would negate the safe tranport
of passengers.

The advantages of this network compared to alternatives such as
rail is that it would be quiet, have extremely low rolling resistance
and the power stations would only need to be built at those points
where elevation needs to be gained. One could presumably do the
same thing with water but that medium is heavy, attracts algae
and mosquitoes, freezes in cold weather and doesn't inspire that
'half-baked' feeling.

Canals is a fundamentally good idea, but water is
heavy. In suitable geography, you just dig a
trench,
and the weight of the water dosn't matter. But in
hilly terrain, you have to build elevated -- thingies
(are they viaducts or aquaducts?), and that's
where
this idea really shines. [+]

re: //A major difficulty...// A third way is to do
this on some other planet. What would be
climate change on Earth would be terraforming on
Mars. Plus, it's just not right that Mars doesn't
have canals, and this would be a way to get those
built.

Tempered glass, if the zeppelin is carrying heavy items the plexiglass might not not hack it strength-wise. Being such a wildly dangerous idea, it gets my vote, despite the absence of either cheese, or stripes.

//six times the density of air//So the airships can be 1/6 the size... maybe larger if you absolutely insist on the passengers breathing.

//terraforming on Mars// the WP article on that is interesting, and it does mention SF6. It'd certainly be an easy way (comparitively) to get Earth-normal pressure. Wonder if you could use octo-oxygen.

I assumed that this was going to be a network of upside-down canals on stilts. These canals could be filled with a lighter-than-air gas such as Helium. Airships would 'float' on these canals and be guided from one place to another.

sorry, all the possible uses are satisfied fine by rail and zeppelin already. Rail isn't THAT loud (for you city-dwellers used to overhead metro lines, the directly-in-ground lines aren't nearly as loud as the ground absorbs the vibrations, frankly the horn is the loud part), and it's infrastructure is cheaper than this cockamamie thing you describe. And zeppelins already provide a leisurely air ride.

PS: the 'horn' of a North American locomotive (it's actually
still called a whistle) blasts at 96-110 db @ 100' from the
source, making it the loudest commercial land
transportation vehicle in the US & Canada (and probably in
the world, but I can't back that statement). The diesel
power plant of a freight
locomotive at full revolutions can exceed 85 db @ 100',
which is also pretty damn loud. Braking and air pressure
release can approach 95 db.

^quizzically^ You say "cockamamie" like it's a Bad Thing?
This confuses me.

Seriously [Edwin], you shouldn't make sweeping statements
on the HB like "all possible uses are already
satisfied by rail and zeppelin". What about the difficulties
caused by rail lines which divide a farmer's field in twain,
separating his milk cows from his rhinoceroses thus
spoiling
his plans for breeding milkers unafraid of wolves? What
about relieving the drain on helium supplies or obviating
the dangers of hydrogen filled airships? Or are you one of
those people who get off on footage of the Hindenburg
disaster? Oh the humanity!

My plan would partially sequestrate the SF6 currently being
emitted by volcanoes thus contributing to the battle
against Global Warming.
What have the polar bears ever done to you? (disclaimer - if
you or a close family member have ever been mauled by
said polar bears then I withdraw that last statement).

Not because of the relative densities of the gasses. A
helium airship floats in air which is 4 times heavier than
helium. My air-filled airship would be floating on a gas 6
times heavier than air so there could be a marginal
reduction in size to achieve the same lifting capacity.
Reality gets more complicated, however, than just the
relative weights of the gasses otherwise hydrogen would
lift twice as much as helium instead of just 13% more.
[link]

The big savings in payload come from not having to carry
the motive power and fuel. The same link shows that the
Hindenburg carried up to 58 tonnes of fuel compared to
only 9.5 tonnes of passengers and payload. No weight is
given for the motors. This would allow for a massive
reduction in airship size relative to payload with my
sulphur hexaflouride flume system.

[NotationToby] you need to amend your link to point to
your post rather than just the category. Perhaps we could
combine our systems where my chutes morph into tubes
when traversing below water bodies.

//Reality gets more complicated// umm... gets simpler ackshully. If you need to displace <x> volume of gas1 to get to neutral buoyancy then, if gas2 weighs 6x's as much you only need displace <x>/6 volume.

I take your point [FT] about displacement and buoyancy
and the lifting capacities of gasses floating on mediums of
different densities. But I wasn't using hydrogen or helium
to fill my airships to float on sulphur hexahydrate but was
using plain old air. Your 6x gain might well hold true if I
used one of the conventional lifting gasses over the 6x
heavier medium of SF6.

I also reassesed some of my own assumptions after I read
the linked article which claims a much diminished lifting
performance gain of only 13% for hydrogen over helium
despite air H weighing 1/14th the weight of air while He is
doubly heavy at 1/7th of air. That is even allowing for the
diatomic nature of hydrogen molecules.

ah right right; I had thought of that but got distracted by something shiny. Call it 5x then eh ? H2 and He are an effective weight of zero so the weight displacement is (1 - 0) volume's worth of air. SF6 is 6x the weight of air so, using air to fill, it's (6-1)'s volume's worth of air.

That statement annoys me every time I see it. It's highly misleading, because it ignores the mass of the airship. In terms of _payload_, hydrogen gives a better advantage than that.

//SF6 is 6x the weight of air so, using air to fill, it's (6-1)'s volume's worth of air//

Not quite. You're forgetting that there's only (about) 1/6 as much air. The actual displacement of an air-in-SF6 craft will in fact be close to 1/6 that of a helium-in-air craft, for a given payload. Possibly even better, because the structure could be lighter, following the cube-square law.

Yes, the numbers are correct, but their application is still misleading. It's the gross mass, rather than the payload, that can be 1.13 times as great with hydrogen compared with helium. For instance, if the payload and the vehicle are of equal mass, then the advantage with hydrogen is not 1.13, but 1.26 - in other words, in that example, hydrogen is 1.26 times as 'good' as helium.

Even if you had a vehicle that weighed literally nothing, so all the lifting is on the payload, then you would get basically 1.26/1.13 or roughly 13% better.

AhI see, you're saying that there's extra volume available to lift the payload, so that a vehicle that was perfectly balanced with Helium and could lift nothing, could lift 13% more of its mass, which is *all* payload, for an increase of infinity%. Got it.

//That would be compared to a vacuum.// Not at all. I'm merely assuming that the dead weight of the vessel (structure, engines, gas bladders, etc.) is approximately constant, so the increased payload is proportionally greater than the increased 'lifting power' (a vague term, which is partly responsible for this confusion) of hydrogen over helium.

Even that is a slight simplification, as it ignores changes in density due to the pressure gradient, but it will give good answers, as opposed to any simple comparison of gas densities, which is as meaningless in isolation as trying to predict the top speeds of two vehicles based solely on cylinder displacement.